1. Field of the Invention
The present invention relates to an aluminum alloy which is used as a core of the clad material used as a passageway for a fluid, such as water, in a brazed assembly, such as a header plate, tube, drawn cup, and the like, of a heat exchanger for automobiles. More particularly, the present invention relates to an aluminum alloy-core material for fluid-passageway use, having improved corrosion resistance and mechanical properties after brazing.
2. Description of the Related Arts
Generally, a passageway for a fluid in a brazed assembly, such as a header plate, tube, drawn cup, and the like of a heat exchanger made of aluminum alloy, such as a heat exchanger for automobiles, consists of: the core material, for which 3003 alloy (Al-Mn series alloy) according to the relevant JIS standard or AA standard (hereinafter, all alloy designations are according to the JIS standard or AA standard) or 6951 alloy (Al-Mg-Si series alloy) is used; and a clad material, which is an Al-Si series- or Al-Si-Mg series-filler metal, or sacrificial anode made of, e.g. 1070 alloy, and which is clad on one or both surfaces of the core material. The fluid passageway consisting of the clad material as described above is bonded with the articles to be brazed, such as the fin materials, thereby forming the brazed assembly.
A number of corrosion protection techniques of materials for the heat exchangers, utilizing a sacrificial anode, are known. But, as shown in Japanese Examined Patent Publications No. 58-23560 and No. 58-24179, most of those techniques propose to use the fin material as the sacrificial anode. The sacrificial anode effect by the fin materials can not be expected, however, when a part of or the whole of the fluid-passageway is positioned far from the fin materials.
In addition, there have been disclosed techniques for cladding the sacrificial anode layer on the fluid passageway at a part thereof in contact with the fluid, for example, as in Japanese Unexamined Patent Publications No. 59-89999 and No. 59-205445. In this case, the sacrificial anode layer is of a specified kind and all other kinds are ineffective. In addition, the corrosion resistance of the sacrificial anode layer seems to be poor on the ambient air side, and the corrosion resistance of the core material per se seems to be poor.
The core of the clad material used as a passageway for a fluid, such as water, in a brazed assembly of a heat exchanger made of an aluminum alloy, must have an excellent corrosion resistance and the mechanical properties required for the brazed structure. The 3003 alloy and 6951 alloy conventionally used as the core material as described above allegedly have a relatively good corrosion resistance and attains a fair degree of mechanical strength. But, in actual use as the core material, the corrosion resistance thereof at a part in contact with the ambient air and a part in contact with various fluids is not always sufficient, with the result that, occasionally, pitting corrosion and intergranular corrosion occur, depending upon the environment, to render the assembly useless. Regarding the strength, since heating to a high temperature is carried out upon brazing when a brazed structure is assembled, softening occurs and causes a reduction of the mechanical properties of core material, with the result that the strength required for the core material is not always obtained. Accordingly, it is clear that a core material which thoroughly satisfies both the requirements for corrosion resistance and strength after brazing, heretofore could not be obtained.
Japanese Unexamined Patent Publication No. 59-89999 proposes, as a core material, to decrease the Cu and Mn of AA 6951 and to add one or more of Cr, Ti, and Zr, to lessen the sensitivity to intergranular corrosion. It also proposes to make a composite cladding material with a Ca addition, to enhance the corrosion resistance of the core material. Note, the composition of the core material is usually 0.2.about.0.8 wt % of Cu, 0.1.about.0.8 wt % of Mn, 0.1.about.0.5% of Mg, 0.1.about.0.6 wt % of Si, and one or more of 0.01.about.0.3 wt % of Cr, 0.01.about.0.3 wt % of Zr, and 0.01.about.0.3 wt % of Ti, and a balance of Al.
The corrosion resistance of core material (the maximum pitting depth) is constant within the above compositional range not depending upon the contents of elements.
In the invention of the publication mentioned above, a calcium-containing aluminum alloy must be used for the inner cladding material of a water chamber, since Fe is an impurity in this invention.
In the case of not intentionally adding Fe, it is possible that Fe is incorporated in the alloy produced industrially in an amount of 0.7% by weight at most. The alloy of this invention is strengthened by Mg and Si, but the resistance against intergranular corrosion and pitting corrosion may not be satisfactory, due to the Mg and Si contents, and a rather high Fe content, concretely exceeding 0.3 wt % of Fe.
As shown for example in Japanese Unexamined Patent Publication No. 59-89999, the corrosion protection of core material due to a sacrificial anode has been heretofore evaluated by an electrode potential of the core material. It has, however, been discovered by the present inventors that such an electrode potential does not provide information about how long the core material can be protected cathodically by a sacrificial anode or filler metal.